In the field of this invention it is known that first and second generation cellular standards all use “Frequency Division Duplex” (FDD) in which there are separate downlink (base station to mobile) and uplink (mobile to base station) frequency allocations. These allocations are separated by a “duplex spacing” to prevent interference between the simultaneous transmission and reception taking place at both the base station and mobile. FDD allocations are typically termed “paired spectrum”.
“Time Division Duplex” (TDD) is used in more recent standards, such as “3rd Generation Partnership Project” (3GPP) “Time Division-Code Division Multiple Access” (TD-CDMA) and 3GPP “Time Division-Synchronous Code Division Multiple Access” (TD-SCDMA). In TDD systems, transmission and reception takes place alternately in time on the same frequency. TDD is very well suited for packet data communication where uplink and downlink capacity can easily be adjusted to meet subscriber traffic profile.
TDD is not used in FDD bands, because of interference concerns. TDD can operate in the mobile transmit (uplink) portion of a FDD band without detrimental interference. The allocation of TDD channels immediately adjacent to the FDD uplink channels in the “International Mobile Telecommunications 2000” (IMT-2000, International Telecommunication Union designated ‘3G’ band) provides evidence of the feasibility of this. The frequency allocation for IMT-2000 is shown in FIG. 1.
However, operation of TDD in the downlink portion of an FDD band is problematic, because of adjacent channel interference from existing FDD base stations to the receivers of co-located or nearby TDD base stations, both of which typically transmit at higher power than the corresponding user terminals.
Consequently, where a wireless operator has an FDD spectrum allocation, TDD technology can normally only be operated in the FDD uplink part of the spectrum, leaving the FDD downlink spectrum unutilized and effectively ‘wasted’.
A need therefore exists for an arrangement, method and unit for TDD operation in a communication system wherein the abovementioned disadvantage(s) may be alleviated.